Drying pigments



April 4, 1939- A. E. VAN wlRT E1- AL 2,152,716

DRYING PIGMENTS Filed Feb. 25, 1937 PRE C /P/ 7H 770A/ ATTORNEYSPatented Apr. 4, 1939 l UNITED STATES PATENT .oF'FlcE name HcMsN'rsApplication February z3, 1937ser1a1No. 127,184

. I9 Claims. 'Ihis invention relates tothe drying of pigments, andparticularly to the drying of pigments which are precipitated fromaqueous solution or occur as a mixture of pigment particles Iand waterat a stage in their process of manufacture. The invention is concernedprimarily with the drying of aqueous mixtures of iron blues, i. e. ironferrocyanide pigments such as Prussian Blue, Steel Blue, Milori Blue,Chinese Blue, 10 Bronze Blue, etc., either alone or in combination withother pigments, suchas the well-known mix-v ture of iron blues with leadchromate called Chrome Green.

Pigments which are precipitated from solu tions, particularly aqueoussolutions, are usually inthe form of very ne particles, which in manycases require no further reduction in size to render them suitable forincorporation into printing inks, enamels, paints, lacquers and othercoating v compositions. When, however, the aqueous pigment pulp issubjected to drying, the pigments manifest-an unfortunate tendency tocake and to form hard aggregates in which several pigment particles arebound together. When this occurs the dried pigments must be subjected tosevere mechanical action by means of ball mills, roller mills'or othertypes of grinders in order to break up the aggregates. Thus the ironblues, which have an extremely fine particle size as originallyprecipitated, form exceedingly hard and tenacious aggregates upondrying, and in heretofore customary practice have been subjected to anardu` ous grinding operation to break up aggregates.

As a result of our investigations we have discovered -a method ofremoving water from Wet pigment pulps which inhibits the vformation ofhard aggregates and leaves the dried pigment in a loosely consolidatedcondition. Marketable dry pigments of sufdcient iineness for use incoating compositions can be prepared from the product of our processwith only a fraction of the heretofore customary grinding.

In accordance with. our invention an aqueous pigment pulpA is preparedfor drying by incor- -porating into it a softening agent in the form ofa volatile liquid 'capable of inhibiting the formation of hard pigmentagglomerates during drying and having a boiling point higher than thatof water at the same pressure. The resulting mix.

ture is subjected to heating to evaporate the water andthe softeningagent from the pigment. Because the boiling point of the softening agentishigher thanthat of Water, thelatter is driven oil ahead of the former,so that the ylast liquid .fraction in contact with the pigment duringdryf ing is substantially all softening agent. This permits thesoftening agent to operate at the crucial time without interference fromthe water.

The vapors of water and softening agent are collected and condensed.Thereafter the softening'agent is separated from the water in thecondensate and added again to the aqueous pigment pulp which is to bedried, thus establishing a cycle in the process. y

Preferably the softening agent is separated l0 'from the condensate by a"salting out process g which involves adding to the condensate a sub- Lstance (hereinafter-called a separating agent) which is soluble in waterand substantially insoluble in the softening agent. The addition of sucha substance in sufiicient quantity causes the condensate to separatein'to two substantially immiscible layers, one containing substantiallyall of the softening agent and the other comprising an aqueous solutionof the separating agent (provided that the solution and thesofteningagent have different specic gravities) Itis then relativelysimple to separate the solution from the v softening agent, for exampleby decantation.

. After the softening agent has been removed, 25 the separating agentmay be recovered from the Water which accompanies it and added again tothe condensate. y

The evaporation of the softening agent and water from the pigment shouldpreferably be conducted under vacuum in order to lower the boilingpoints of 4both softening agent and water. Preferably the degree ofvacuum should be sumcient to permit the removal of -all liquid from thepigment without exceeding a temperature of about C. Such practiceminimizes the possibility of deleteriously affecting the pigments by.

- heating them to an excessive temperature.

The softening agent employed should meet the following qualifications: l.4

1. It must have the faculty of inhibiting the formation of hardagglomerates in the particular pigment undergoing drying in contact withit.

2. It must be relatively volatile, lbut must have a boiling pointabove-that of water at the pressure under whichevaporation is conductedso that the water will evaporate in Iadvance of the softening agent.Moreover,l the lower is the boiling point of a softening agent, the morediilicult it is to condense and the greater will be the oppor- 50vtunity for loss of the softening agent as vaporfrom the condenser. y

3."1he softening agent should be such as to v withstandrepeatedevaporation without substantial decomposition, i. e., stable.

l-4. Itshould be substantially chemically inert with respectv to water,the pigment, and other materials in which it comes in contact in theoperation of the process.

5. The pigment should be substantially insoluble in the softening agent.

6. The softening agent should be volatile (at the pressure at whichevaporation is effected) at a temperature below that at which thepigment begins to decompose or to be otherwise deleteriously affected.

7. The softening agent should be easily and cheaply-separable fromwater, but may be miscible with water.

From a practical standpoint, the following are desirable characteristicsIof the softening agent,

- because in practical operations-a small amount of the softening agentmay be left in the dried Pigment through accident or design:

8; Pleasant odor. 9. Non-toxicity. l0. Compatibility with oils and othervehicles in which the dried pigment may be incorporated. 11.4 Thesoftening agent should not decrease the WetabilityA `fof dried pigmentswith the vehicles in which they are to be incorporated.

In the case of the iron blues, a softening agent which fulfills all oftheforegoing qualifications is mono-butyl ether of ethylene glycol. Thiscom- 4 pound boils at a temperature in excess of the boiling point ofWater. Its boiling point at atmospheric pressure is about 165 C.However,

under an easily attainable vacuum, correspond-l ing to say 28 inches ofmercury, the compound boils at a temperature below 100 C. Itselimination, therefore, does not involve high temperaa suitablevseparatingl agent.

` ethylenev glycol have proved to be`desirable, al-

tures which might result in damage to the pigment. 'I'he mono-butylether of ethylene glycol is stable, water-miscible, chemically inertwith respect to water and to the iron ferrocyanide pigments, and iseasily separated from water either by fractionation or by decantationafter adding 'I'he iron ferrocyanide pigments are substantiallyinsoluble in this compound, which moreover is harmless, has a pleasantodor, is soluble in most oils and water and does not ailect adverselythe wetability of pigments by vehicles. If, therefore, a small amount ofthe agent should be left upon the pigment in the drying operation,v itspresence would The homologues of monobutyl ether of ethylene glycolwhich have a boiling point higher than 100"` C. at atmospheric pressuremay also be employed to advantage in the practice of our invention withthe iron blues. Of the homologues the mono-methyl ether and themono-ethyl ether of though neither are so satisfactory as their butylhomologue. y I f These and other features of our invention will be morecompletely understood -in the light of 4the following detaileddescription 4of a presently preferred practice of the invention inremoving water from anaqueous pulp of iron blue pigment,

'taken in conjunction with the accompanying single ligure, which is adiagrammatic ow sheet of the preferred practice.

Referring now to the ilow sheet, it will be seen that an aqueous pulpofiron'blue pigment isvdearcane upon the character of the pigmentparticles, theV lforce exerted in filtration, etc.

It is possible, of course, to treat the raw pulp by the process of theinvention without first partially dewatering it.v However, in mostinstances it will be more simple and economical to first remove part ofthe water by filtration, settling or even by partial evaporation,providing that substantially none of the pigment particles becomecompletely dry so as to be bound into aggregates.

-The wet pulp of iron blue containing about 70% ofy water and 30% byweight of pigment isA mixed with about 40% by weight of a softeningagent, such as ethylene glycol mono-butyl ether. The mixture is thenplaced in 'a drier, preferably a vacuum evaporator provided with' aninternal agitator. It may be convenient to mix the wet pulp and thesoftening agent within the vacuum 'evaporator itself. After thoroughmixing, heat is applied to the vacuum drier and the vapors of water andof the softening `agent are driven off under vacuum. A relatively highdegree of vacuum is desirable, so that a low temperature of evaporation(preferably below 100 C.) will result in freeing the pigment of allliquid. Thus, when employing the mono-butyl ether of ethylv ene glycolas a softening agent a vacuum corresponding to .a manometer reading of28 inches of mercury should be maintained,l in whichcase drying iscompleted the liquid remaining in con-` tact with the pigment is almostentirely softening agent. The drying operation is continued under h?dvacuum until the pigment is substantially The dried pigment resultingfrom this opera'.- tion is `soft and relatively unconsolidated.v It iswithdrawn from the drier and subjected to further processing, ifdesired. A relatively slight grinding operation will render the driedpigment sufdciently 'i'lne for use in practically all coatingcompositions. 1

The vapors of water and of softening agent driven off from the vacuumdrier are sucked out of the drier by means of a vacuum pump andcondensed by cooling. The condenser may be of any of the conventionaltypes in which liquid employed'forcooling does not become mixed with thecondensate. A satisfactory form is a simple cooling coil immersed in abath-through whichl cold water is circulated, the condensation takingplace within the coil. Preferably the condenser is disposed between thevacuum drier and the vacuum pump so as to minimize the load upon thelatter. It is possible, however, to place the condenser in the systemfollowing the vacuum pump, in which case the vapors pass through thevacuu pump and are later condensed.

I'he condensate is a clear homogeneous liquid because the softeningagent and the water are mutually soluble, i. e. miscible with eachother.

. tillation, fractional condensation or by the use of than water,

. least a portion of said vin an economical manner a rectifying column.However, we have found i that a much simpler and wholly satisfactoryseparation of the softening agent from -the water maybe achieved byadding to the condensate a separating agent in the form of a substancewhich is soluble in water but substantially insoluble in the softeningagent. Thus, in the case of a condensate containing mono-butyl ether ofethylene glycol and water, separation is achieved by adding theretosodium sulfate, preferably in the anhydrous form and in amountsuillcient to saturate the water.. If anhydrous sodium sulfate isthoroughly incorporated into the 'condensate and thereafter thecondensate come quiescent, the condensate will separate into twosubstantially immiscible layers. The upper layer contains about of thesoftening agent and about 10% water substantially free from sodiumsulfate. 'Ihe lower aqueous solution of sodium sulfate substantiallyfree of the softening agent. The two layers are separated from eachother by simple decantation as shown on the .flow sheet.

Other stable salts which are 4 soluble in water and substantiallyinsoluble in the softening agent may be used instead of sodium sulfate.

The decanted upper layer consisting principally of softening agent isreturned and used over again by mixing it with the pigment pulp prior todrying.

The separating agent, such as sodium sulfate, is recovered very simplyby subjecting the decanted lower layer to boiling, whereupon the lwateris evaporated and the sodium sulfate is made available for re-use and isreturned to be mixed with the condensate. The boiling may be conductedat atmospheric pressure;

f To hasten or promote the drying of the'pulp a tertiary liquid, such asalcohol, benzene, or other substance having a lower-boiling point thanthe softening agent, may be added to the pulp together with thesoftening agent prior to drying. Preferably the tertiary liquid isinsoluble in the softening agent, thereby facilitating subsequentseparation of the softening agent from the condensate.

By recovering the softening agent or the separating agent or both asvdescribed hereinbefore and using them over again in a cyclic manner,the process of the invention may be conducted )and with relatively smallloss of the agents.

We claim: v l. In a process for removing water from wet ironferrocyanide pigment, the improvement which comprises incorporating inthe wet pigment a liquid selected from the group consisting ofmono-butyl ether of ethylene glycol and its watermiscible homologueshaving aboiling point higher and removing water and at least a portionof said liquid from the wet pigment by evaporation.

2. A process for removing water from wet pigment which comprisesincorporating in thewet pigment a liquid selected from the groupconsisting of mono-butyl ether of ethylene glycol and its water-misciblehomologues having a boiling point higher than water and removing waterand at by evaporation'. 'J

l 3. A cyclic process iron ferro-cyanide pigments for removing waterfrom and inhibiting cakis permitted to be` layer is a saturated liquidfrom the wet pigment v ing thereof which comprises incorporating in amixture of the pigment andwater an organic liquid ofthe group consistingof mono-butyl ether 4. A cyclic process for removing water from`wetpigment which comprises incorporating in the wet pigment a liquidselected from the group consisting of mono-butyl ether of ethyleneglycol and its water-soluble homologues having a boiling point hi herthan water, subjecting the resulting mixt re to evaporation to removewater and at least a portion of said liquid from the pigof ethyleneglycol and lits water-miscible homoment as a vapor, condensing thevapor, adding Y a salt that is soluble in Water and substantiallyinsoluble ,in the liquid to the resulting condensate to cause the waterand the liquid to become substantially immiscible, thereafter separatingthe liquid from the water and the salt and re- .turning the liquid tothe pigment to be utilized again.

5. A cyclic process for removing water from pigment and inhibitingcaking thereof which comprises incorporating in a mixture of the pig'lment and water an organic substance of the group consisting of themono-butyl ether of ethylene glycol and its lwater-miscible homologueshaving a boiling point higher than water, heating n the resultingmixture to drive off' vapor containing water and the organic substance,condensing the vapor, adding to the resulting condensate a salt that isywater soluble but substantially insoluble in the organic substance tocause the water and the organic substance to separate into substantiallyimmiscible layers, thereafter separating the'water and the organicsubstanceby decan- '1. A process for removing water from iron a mixtureof the pigment and water, re,

ferro-cyanide pigments which comprises incor l porating normal ethylether of-ethylene glycol in a mixture of the pigment and water andsubjecting the resulting mixture to drying to evapcrate the water andthe normal ethyl ether of ethylene glycol. j

8. A process for removing water from iron ferro-cyanide pigments whichcomprises incurporating in a mixture of the pigment and'watermono-methyl ether of ethylene glycol and sub.

:lecting the resulting mixture to drying to evapf crate lthe water andthe lmono-methyl ether of ethylene glycol. l l9. Acyclic process forremoving water from aqueous pigment which comprises incorporating vinthe aqueous pigment a. relativelyv v olatile organic water-miscibleliquid of the pigment during dryin higher than water, is chemio -inertwith rethat inhibits caking has a boiling point u 4 amiamo spect toWater and the pigment, will withstand repeated evaporation withoutdecomposition, and in which the pigment is substantially insoluble,subjecting the resulting mixture to evaporation to drive oi water andthe organic' liquid as a vapor, condensing the Vapor, adding to theresulting condensate an anhydrous salt which -is water-soluble butsubstantially insoluble in the .organic liquid thus forming, an aqueoussalt solution Whichimmiscible with and of different density than theorganicl liquid, separating the organic` liquid from the salt solutionvby deantation and returning the organic liquid to the aqueous pigmentthereby utilizing the organic 5 liquid again.

` ALFRED E. lVAN WIRT.

ANDREW G. AYLIES.

' GEORGE F. JONES.

